The Inner (Alimentary) Tube and Its Respiratory Derivatives 53 



Especially important is the differentiation of the form of the 

 tooth in adaptation to a diversity of diets and uses. The sharp- 

 pointed, backward-curving teeth of fishes serve primarily for seizing 

 and holding prey. In amphibians and most reptiles, the teeth retain 

 this primitive function. But, especially in mammals and occasionally in 

 other vertebrates, teeth acquire bladelike or chisel-shaped crowns 

 (secodont teeth) adapted for sharp cutting; or they become massive, 

 with crowns flat-topped and transversely corrugated (lophodont 

 teeth) adapted for grinding; or certain front teeth may become much 

 elongated, sharp-pointed fangs or tusks serving for holding prey, tear- 

 ing flesh, digging, or as weapons of combat. The large "cheek-teeth" 

 of mammals are especially complex in form. The crown may bear two, 

 three, or more projecting cusps or tubercles (bunodont teeth adapted 

 for crushing — bicuspid, tricuspid, triconodont, tritubercular, 

 multi tubercular), and the tooth may have two or more roots (Figs. 

 42, 43). 



The evolutionary origin of these large, complex teeth of mammals 

 is a problem for whose solution two opposed theories have been offered. 

 The concrescence theory holds that the multiple cusps and roots 

 indicate origin of the tooth by fusion of two or more primitive simple 

 conical teeth. The theory is supported by some evidence that the large 

 mammalian molar develops by fusion of two or more embryonic tooth- 

 germs. The differentiation theory, strongly supported by paleonto- 

 logic evidence, maintains that the mammalian molar has been derived 

 from a single primitive tooth by progressive increase in size and 

 complexity. 



In most fishes and in amphibians, the teeth of an individual animal 

 are of the same form in all regions of the jaws — the dentition is homo- 

 dont. In some of the more primitive reptiles and in nearly all mam- 

 mals, the individual dentition becomes locally differentiated so that 

 the teeth in one region of tiie jaw may perform a function quite unlike 

 that of teeth in another region. The animal may bite or cut with blade- 

 like front incisors, crush with massive cheek-teeth, and grasp, tear, or 

 fight with conical canine tusks. This heterodont condition reaches its 

 extreme development in mammals (Figs. 43, 53). 



Replacement of Teeth. Another marked difference in verte- 

 brate teeth is in the frequency of their replacement. The shark has a 

 continual stream of teeth flowing (slowly) across the jaw — new teeth 

 forming on the lingual side and becoming old as they slide across to 

 the labial side to be eventually shed (Fig. 50). In other fishes and in 

 amphibians, the replacement of teeth occurs in a relatively desultory 

 manner, but apparently a tooth in a certain location may, from time 

 to time, be shed and replaced for an indefinite number of times. In 

 case of a tooth situated on the jaw, the replacing tooth develops on 



